EP0387096A2

EP0387096A2 - Image forming method

Info

Publication number: EP0387096A2
Application number: EP90302563A
Authority: EP
Inventors: Masahiro C/O Intellectual Property Div. Hosoya; Mitsunaga C/O Intellectual Property Div. Saito; Yoshimitsu C/O Patent Division Ohtaka; Mitsuharu C/O Patent Division Endo; Yukio C/O Patent Division Futamata
Original assignee: Toshiba Corp; Tokyo Electric Co Ltd
Current assignee: Toshiba Corp; Toshiba TEC Corp
Priority date: 1989-03-10
Filing date: 1990-03-09
Publication date: 1990-09-12
Anticipated expiration: 2010-03-09
Also published as: US5051332A; EP0387096B1; KR930005906B1; DE69004713D1; EP0387096A3; DE69004713T2; KR900014940A

Abstract

An image forming method is to arrange a developing agent holder for holding one component developing agent layer arranged to contact with a latent image holder, and to develop certain latent image on the latent image holder and to simultaneously clean up the developing agent adhering to the surface of non-latent image area on the latent image holder, wherein the cleaning up is carried out under the condition expressed by the following formula:
0.5≦ (Vd/Vp) · m ≦ 3.0
wherein the moving speed of the developing agent holder is defined as Vd, the moving speed of the surface of the latent image holder is defined as Vp and the developing agent adhering density is defined as m (mg/cm²),
or the amount of the remained toner after transferring remaining on the latent image phase of the latent image holder is set less than 0.35 mg/cm². Further, an image forming device is provided with a remained toner uniforming means for disturbing and uniforming the remained toner remaining on the latent image phase of the latent image holder arranged to be pressed or contacted to the latent image holder for being applied to the above mentioned image forming method. Therefore, the satisfactory image having an excellent quality without ghost and fogging can be always obtained, and the satisfactory image can be obtained under the high humidity environment condition.

Description

The present invention relates to an image forming method depending on the electrophotographic process or the electrostatic recording process, and more particularly relates to a cleanerless image forming method which is capable of forming an image without cleaning device for cleaning a toner remained after transferring the image.
In general, an image forming method as shown cross sectionally its summary of the structure in Fig. 1 is used, for example, in the device for imaging a latent image (image), such as the electrophotographic apparatus or the electrostatic recording apparatus. Usually, the apparatus provided with a latent image holder, for example, a sensitive drum 1, an electrification device 2 arranged on the periphery of the sensitive drum 1, an exposure device 3, a developing device 4, a transfer device 5 and a cleaning device 6 having a cleaning blade 6a is used. Therefore, an electrostatic charge is carried out by the electrification device 2 on the phase of the sensitive drum 1, and the selective exposure is carried out by the exposure device 3 in the electrification area to form an electrostatic image in the apparatus mentioned above. And, the toner is selectively adhered to the formed latent image area (after the toner image is formed) by the developing device 4, then, the obtained toner image is transferred on a transfer paper 7 in the transfer device 5. The remaining toner on adhering to the phase of the sensitive drum 1 is removed by the cleaning blade 6a of the cleaning device 6.
However, in the apparatus described above, the cleaning device 6 is required to be separately provided to the position opposing to the developing device 4. Therefore, the limitation is caused on the arrangement of the electrification device 2, the exposure device 3, the transfer device 5 and other devices required for forming certain images. Further, the flexibility on the design of the devices is decreased. And a surface of the sensitive drum 1 is worn by the cleaning blade 6a to cause disadvantages, such as the deterioration of the characteristics and the decrease of the service life, during the cleaning operation. Further, the ozone product is generated during electric charging in the electrification device 2, thereby, a negative electrode organic photo conductor (referred to as "OPC", hereinafter) is deteriorated. Therefore, the surplus ozone products are required to be exhausted immediately. However, it is caused a problem that an exhaust path for the ozone products is inhibited by the cleaning device 6. Further, the toner retrieved in the cleaning device 6 is required to be suitably discarded. Therefore, it is caused problems that the maintenance becomes complicate and the peripheral devices are possibly suffered and stained.
In view of the problems mentioned above, it has been developed a device or a means having an exclusive cleaning device for carrying out a developing at the first rotation of the sensitive drum 1 by means of the developing device and for cleaning at the second rotation by means of the same developing device 4, as described in Japanese Patent Application Laid-Opened Official Gazette SHO-62-211681. However, in the case mentioned above, since the developing step and the cleaning step are carried out separately, it is required a sensitive drum 1 having longer periphery than the image length to be formed, instead of the exclusive cleaning device is not required to be arranged. Therefore, it cannot be avoided to be formed not only the sensitive drum but also the whole device in a large size.
The image forming device (referred to as "cleanerless image forming device" hereinafter) for retrieving the remained toner simultaneously with developing an image by means of the developing device without using the cleaning device described, for example, in Japanese Patent Application Laid-Open No. 133573, 1984 and Japanese Patent Application Laid-Open No. 157661, 1984, has been known. In the official gazettes as described above, the basic conception of the cleanerless image forming device is disclosed, and the essence thereof can be summarized as described below. The main construction of the well-known reversal developing method is shown cross sectionally in Fig. 2. This reversal developing method is used in many cases in the electrophotographic printer, such as the laser printer standing therefor. In the reversal developing method, the toner particles 8 charged to have similar polarity as a latent image holder, for example, the sensitive drum 1 are used to be adhered to a portion on the sensitive drum where the electric charge is not existed (or existing in small amount). Herein, the toner particles 8 are not adhered to the portion where the electric charge is existing. For achieving such selective adhesion of the toner, the voltage V_b(|V|<|V_b|<|V₀|) between the electric potential V₀ of the charged portion and the electric potential V of non-charged portion on the surface of the sensitive drum 1 is applied to a toner holder 4a (developing agent holder) in the developing device 4. And the adhesion to the sensitive drum 1 is controlled by the electric field between the charged portion. Then, the toner 8 is adhered to the sensitive drum 1 by the electric field between the non-charged portion. The toner 8 adhered to the sensitive drum 1 is transferred to an image supporter 7 by means of the well-known transfer device 5. During the transfer step, the whole toner is not transferred in general, and the remained toner 8′ after transfer is existing on the surface of the sensitive drum 1 in the image form. In the usual image forming device, for example, the electrophotographic device, the remained toner 8′ is retrieved by means of the cleaning device 6 shown by the dashed lines. Then, the electric charge on the surface of the sensitive drum 1 is removed by means of an electric removal lamp 9, and it is returned to the latent image forming step (a uniform charging step by the charging and an exposure step by the exposure device 3). In the cleanerless image forming device, the remained toner 8′ is transferred to the developing step without using the cleaner device 6 and is retrieved in the developing device 4 simultaneously with developing. Strictly, since the remained toner 8′ existing in the charged portion (non-exposured part or non-imaged part) in the latent image formed by the exposure of the exposure device 3 is certainly charged to have same polarity as the latent image by means of the electrification device 2, it is transferred to the toner holder 4a side by means of the electric field (electric field caused by the potential difference between V₀ and V_b) for controlling the transfer of the toner 8 from the toner holder 4a to the sensitive drum 1. Simultaneously, the remained toner 8′ existing in non-charged portion (namely, the exposure part or the image part) is affected by the force toward from the toner holder 4a to the sensitive drum 1 to be remained on the surface of the sensitive drum 1. The newly supplied toner particles 8 is transferred from the toner holder 4a to non-charged portion, thereby, the cleaning is carried out simultaneously with the developing.
As described above, since the cleaning device 6 and the waste toner box are not required in the cleanerless recording device, the miniaturization and the simplification of the device can be facilitated. Therefore, the merits as described below can be obtained. Since the remained toner 8′ retrieved in the developing device 4 can be reused, it becomes economical without causing the waste toner. Since the sensitive drum 1 is not worn by the cleaning blade 6a, it can be obtained longer service life of the sensitive drum 1.
However, in the cleanerless image forming device, a ghost image is possibly caused by the following reason.
First, under high humidity environment, since the paper as the image supporter 7 takes the moisture to be low resistance, the transfer efficiency becomes lower. Therefore, a lot of toner particles tend to be remained on the surface of the sensitive drum 1. When the amount of the remained toner 8′ becomes in the excessive amount, it cannot be completely cleaned up in the developing device 4. Therefore, the remained toner 8′ stays on non-imaged part to cause a positive ghost on a white portion of the transfer image (referred to as "positive ghost" or "positive memory" hereinafter).
Second, when the amount of the remained toner 8′ becomes in the excessive amount, the light beam is intercepted by the remained toner 8′ during the exposure step by the exposure device 3, thereby, the damping of the electric potential on the surface of the sensitive drum 1 is resulted unsufficient to be as the electric potential condition (referred to as "V_ℓ ′") in the intermediate between V₀ and V_ℓ . In the portion as described above, the developing voltage becomes as V_b - V_ℓ′, which is smaller than the developing voltage V_b - V_ℓ of the periphery exposure part. Therefore, the toner transfer amount from the toner holder 4a to the Sensitive drum 1 becomes smaller as compared with the periphery, thereby, the remained toner image is appeared on the developing part of the transfer image as a void image (referred to as "negative ghost" or "negative memory", hereinafter). This phenomenon notably appears especially in the half-tone image formed of the aggregation of the net point image and the line image, etc.
As compared with this, Japanese Patent Application Laid-Open No. 203183, 1987, discloses to remove the ghost by applying a voltage to an electroconductive brush 10 having such formation as shown cross sectionally in Fig. 3 to be contacted slightly with the sensitive drum 1. Namely, the voltage having the reverse polarity to the electro static charge of the toner is applied to the electroconductive brush 10 by the direct current power, and the remained toner 8′ is absorbed at one to the brush 10 by the Coulomb force. Therefore, the amount of the remained toner 8′ on the surface of the sensitive drum 1 can be remarkably decreased, and it can avoid to cause the above mentioned ghost.
However, in the case of the above system, under the experiment by the present inventor, the deterioration of the cleaning characteristics is often found according to the amount of one component developing agent layer formed on the developing agent holder 4a and other developing condition when the development cleaning is carried out on using one component developing agent. Further, it becomes clear that the sufficient condition carrying out only the developing cannot be always applied thereto. Namely, the paper as the image holder 7 holds (absorbs) much moisture under high humidity condition, therefore, the resistance ratio is remarkably decreased. As a result, the electric charge provided from the transfer device 5 to the paper 7 moves to the thickness direction of the paper 7 to reach the toner particles on the surface of the sensitive drum 1, thereby, the toner is charged in the reverse polarity to the essential electro static charge. Since the toner charged in the reverse polarity is affected by the resiliency caused by the electric field even if contacting with the electroconductive brush 10, it is not absorbed by the electroconductive brush 10. Further, the dispersion of the remained toner image 8′ can keep substantially the original condition after passing through the brush. Therefore, it cannot be avoided to cause the above mentioned ghost in such case.
Further, since the amount of the toner which is able to be held in the electroconductive brush 10 has the limitation, the toner is naturally expelled to the surface of the sensitive drum 1 when it becomes in a certain amount. The expelled toner is not dispersed in a image form not like the remained toner 8′ and it shows the remarkably uniformed dispersion condition, therefore, the above mentioned ghost is not induced. However, in the case in which the solid image is sequentially output (sequential development of the solid image), a lot of amount of the toner is held in the electroconductive brush 10 to cause the possible expulsion of the toner to the surface of the sensitive drum 1. In such case, the above mentioned ghost is generated.
Because of these problems as mentioned above, the image forming by the conventional cleanerless image forming method is hard to carry out under the high humidity environment condition. Further, it has been caused a disadvantage that the property of the image capable of being output has the limitation.
Therefore, the first object of the present invention is to provide the image forming method which is capable of obtaining always the satisfactory image without ghost and fogging by carrying out certain developing using one component toner (developing agent) and simultaneously cleaning up efficiently the remained and adhered toner on the surface of the sensitive body.
Further, another object of the present invention is to provide the cleanerless image forming device which is capable of outputting satisfactory image under the high humidity environment condition and capable of outputting any kinds of images.
An image forming method of the present invention is to arrange a developing agent holder for holding one component developing agent layer arranged to contact with a latent image holder, and to develop certain latent image (imaging part) on the latent image holder and to simultaneously clean up the developing agent adhering to the surface of non-latent image area (non-imaging part) on the latent image holder, wherein the cleaning up is carried out under the condition expressed by the following formula:
0.5≦ (Vd/Vp) · m ≦ 3.0
wherein the moving speed of the developing agent holder is defined as Vd, the moving speed of the surface of the latent image holder is defined as Vp and the developing agent adhering density is defined as m (mg/cm²),
or the amount of the remained toner after transferring remaining on the latent image phase of the latent image holder is set less than 0.35 mg/cm². Further, an image forming device of the present invention is provided with a remained toner uniforming means for disturbing and uniforming the remained toner remaining on the latent image phase of the latent image holder arranged to be pressed or contacted to the latent image holder for being applied to the above mentioned image forming method. By the present invention, the satisfactory image having an excellent quality without ghost and fogging can be always obtained, and the satisfactory image can be obtained under the high humidity environment condition.

Fig. 1 is a cross-sectional view showing the main portion of an image forming device provided with a cleaning device.
Fig. 2 is a cross-sectional view showing the main portion of a cleanerless image forming device.
Fig. 3 is a cross-sectional view showing the main portion of another cleanerless image forming device.
Fig. 4 is a cross-sectional view showing the main portion of a cleanerless image forming device applied to an image forming method according to the present invention.
Fig. 5 is a partially cut perspective view showing a construction of a developing agent holder (developing roller) provided in a cleanerless image forming device used in the image forming method according to the present invention.
Fig. 6 is a typical view for explaining an image forming mechanism.
Fig. 7 is a typical view showing a modeled distributing condition of an electric potential of each portion and a toner density in a development area.
Fig. 8 is a curve view showing a relation between an amount of the remained toner after transferring in the image forming and an amount of the remained toner on the latent image holder after cleaning simultaneously carried out with the development.
Fig. 9 is a cross-sectional view showing the main portion of another cleanerless image forming device used in the image forming method according to the present invention.

Example 1:

First, a function on forming a latent image on a latent image holder and cleaning up simultaneously a toner remained to adhere to the latent image holder will be explained.
On arranging a developing roller for holding a toner particle layer charged to have similar polarity to a latent image electric charge to oppose to a sensitive drum as the latent image holder and by applying certain bias voltage to the developing roller, the development is carried out by an electric field formed in a low electric potential portion (exposure portion), and simultaneously the toner remained and adhered to the surface of the sensitive drum is removed (cleaned up) by the electric field in a high electric potential (non-exposure portion). Herein, it is important that the certain electric field is to formed between the developing roller and the latent image for removing the cleaning up the remained toner on the surface of the sensitive drum. Namely, when the adhered toner layer on the surface of the developing roller is formed in thick, the electric field inhibiting the absorption of the remained toner is formed by the toner layer, therefore, the resiliency affects the remained toner charged in similar polarity thereto. When the adhering amount of the toner on the surface of the developing roller is large, the cleaning of the remained toner is not carried out. Therefore, the ghost image or the fogging is caused by the remained toner in the developed image. As compared with this, when the adhering amount of the toner on the surface of the developing roller is small, the cleaning is effectively carried out, but the sufficient image density cannot be obtained because the amount of the toner adhering to the latent image part (image part) is small. Therefore, the problem on the amount of the adhering toner on the surface of the developing roller can be essentially solved by utilizing the moving speed ratio Vd/Vp of the developing roller and the sensitive drum. Namely, the amount of the toner supplied to 1 cm² on the surface of the sensitive drum in one second can be expressed as Vd/Vp·m (mg/cm²) when the toner adhesion amount (adhesion density) on the developing roller is defined as m (mg/cm²). Therefore, the thickness of the toner layer existing at the developing position can be considered as (Vd/Vp) times of the toner layer apparently formed to adhere to the surface of the developing roller. The thickness of the toner layer (effective toner adhesion amount) is required to be set in a certain range for obtaining really effective cleaning mechanism.
In the present invention, the effective toner layer thickness and/or the effective toner adhesion amount Vd/Vp·m is set in not less than 0.5 (mg/cm²) and not more than 3.0 (mg/cm²) to form certain cleaning electric field between the high electric potential part (non-exposure part) and the developing roller, and the cleaning is efficiently carried out and the sufficient amount of the toner adhesion can be obtained simultaneously in the low electric potential part (exposure part) of the latent image. Thereby, the sufficient developed image having high density can be easily obtained without ghost and fogging.
Next, an example of the present invention will be explained with reference to Figs. 4 and 5. Fig. 4 is a view showing cross-sectionally the main portion construction of the image forming device used for the enforcement of the method according to the present invention. A developing device 4 is provided with a toner container 12 for storing one component toner 8, a toner supply roller 13 for supplying the one component toner 8 to a developing roller (developing agent holder) 4a, a coating blade 14 for forming substantially uniformed toner layer to control the supplied toner on the developing roller 4a, a sensitive drum 1 contacting with the developing roller 4a holding the toner layer to rotate for visualizing a latent image held on its surface, a recovery blade 15 for recovering a remained toner 8′ to the toner container 12, an agitator 16 for agitating the toner 8 stored in the toner container 12 and a spring 17 for pressurizing the coating blade 14 to the developing roller 4a with a constant load.
The material of the sensitive drum 1 can be used, for example, selenium, cadmium sulfide, zinc oxide, amorphous silicon and organic type, and the organic sensitive body is used in the present example. The sensitive drum 1 of the present example is uniformly charged in negative by means of a scolotron charging device 2, and exposed by means of a light beam, for example, a laser beam from a exposure device 3 which is image modulated, thereby, a certain electrostatic latent image is formed on its surface. This electrostatic latent image is visualized by means of the developing roller 4a as mentioned above, thereby, the toner image is formed. Therefore, the toner image formed as mentioned above is transferred to a transfer paper, as an image holder by means of a transfer charger (transfer device) 5 and is fixed by means of a fixing device. Here, the toner 8 is not partially transferred and remained on the surface of the sensitive drum 1 (referred to as "remained toner 8′, hereinafter). This remained toner 8′ is usually removed by means of a cleaning blade. However, in the case of the present invention, the developing device 4 has the function so as to act as the cleaner. Namely, the remained toner 8′ on the sensitive drum 1 is recharged by the charging device 2 after it is aimed by means of an electro removal lamp 9. At this moment, the remained toner 8′ is also certainly charged to have similar polarity with the surface of the sensitive drum 1, and the forming of the electrostatic latent image and the visualization on the image treatment are repeated. During these steps, the remained toner can be retrieved in the developing device 4 by the principle as mentioned below. Namely, on defining the electro potential of non exposure part as Vo and the electro potential of exposure part as Vq among the electro potential on the surface of the sensitive drum, and the developing bias voltage Vb applied on the developing roller 4a through a protection resistance 11a is defined by means of a direction current electric power 11′. Further, the electro potential on the surface of the developing roller 4a (effective developing bias) Ve is set to be similar to the developing bias voltage Vb, thereby, the electrostatic latent image is reversally developed by the one component toner charged in the negative polarity. In the reversal developing, the effective developing bias Ve is set to satisfy with |Vo|>|Ve|>|Vq| (wherein each Vo, Ve, Vq is negative), the development is carried out by the electric potential difference |Ve-Vq|, and the control of the toner adhesion to the non-image part (control of fogging) is carried out by the electric potential difference |Vo-Ve|. In the present example, negatively charged remained toner 8′ is adhered to the surface of the sensitive drum 1, and the remained toner 8′ existing on the non-exposure part (non-image part) is affected by the suction force caused by the electric potential difference |Vo-Ve| at the development position and is transferred to the surface of the developing roller 4a arranged at higher electric potential side (positive electric potential side). As compared with this, in the exposure part (image part), the development is carried out by the action of the electric potential |Ve-Vq| as the remained toner 8′ is being adhered, then, the toner is transferred from the surface of the developing roller 4a to the surface of the sensitive drum 1. When the development of the exposure part is carried out, the remained toner 8′ on the non-exposure part is retrieved in the developing device 4 at the same time.
Further, the explanation will be regarded to the construction and/or the component member of the developing device 4, and the developing roller 4a is constructed as described below. Namely, as shown perspectively the partial cut portion in Fig. 5, a flexible layer 19 and the surface conductive layer 20 are coaxially arranged in order to take the electro conductive shaft 18 as its center shaft, and the surface conductive layer 20 is arranged to extend in the end phase side of the developing roller 4a to communicate with the electro conductive shaft 18. The surface of the developing roller 4a and the electro conductive shaft 18 hold electrically the conductive condition. Therefore, the developing roller 4a is structured to have an electric resistance between the surface thereof in 1 cm² and the electro conductive shaft 18 being set in not more than 1 x 10⁹ Ω · cm², preferably not more than 1 x 10⁷ Ω · cm².
The definition of the resistance R of the developing roller in the present invention is as follows. Generally, specific resistance ρ is used as a resistance value of a substance. The product ρ · ℓe (=R) of the specific resistance value ρ by the thickness of the flexible layer ℓe is used as a roller parameter on which the developing characteristics practically depend. However, practically, the electrode having an area S is contacted on the surface of the developing roller, and the ammeter is connected to the electrode. The resistance value R₀ (=10/I) is calculated from the electric current value (I) measured after applying a voltage of 10 V to the shaft. Furthermore, the resistance value R is obtained from R = R₀ · S. R₀ · S = ρ · ℓe is established, by using the general formula for the definition of the resistance value, R₀ = ρ · ℓe/S. Thus, the roller resistance value R (= ρ · ℓe) of the present invention can be calculated to be equal to R₀ · S.
Further, in the developing roller 4a, the surface conductive layer 20 is required a conductivity, a wear resistance, a chemical stability and an adhesive property with the flexible layer 19. Therefore, the surface conductive layer 20 is formed by coating a compound prepared from an elastomer or a resin, such as polyurethane, polyester, tetrafluoroethylene, polystyrene, acrylic and silicone with electro conductive carbon, metal powder or metallic fiber contained to be dissipated to have the resistance value of not more than 10⁸ Ω · cm, preferably not more than 10⁶ Ω · cm with a spray method or a dipping method on the surface of the flexible layer 19, or by covering the electro conductive tube formed of the above mentioned prepared compound with the surface of the flexible layer 19. The chamfering of about C 0.2 through C 3 or the R working of about R 0.2 through R 3 is preferably carried out previously on both end portions of the conductive layer 19. If the working is done to obtain such form, the possible wearing and peeling of the conductive layer 20 formed on the surface of the flexible layer 19 for the end portion or the end phase of the developing roller 4a can be avoided. Therefore, the conductivity with the electro conductive shaft 18 can be kept for a long period of time. For example, even under the printing test for printing hundred thousand (100,000) sheets having A4 size, satisfactory development was possible to be obtained.
Next, an example in which an electro conductive urethane elastomer is coated on the developing roller 4a will be explained.

(Example 1 of developing agent holder roller)

A coating material combined with an urethane type electro conductive elastomer coating "Electro pack Z-279" (the trade name, manufactured by Taiko Kako Co., Ltd.), a non-yellowing isocyanate type curing agent and a thinner as a diluent being combined in the rate of 10:1:2 was prepared. And the prepared electro conductive coating material was coated on the flexible layer and the end phase on preparing a roller base formed to be coated coaxially with the flexible layer on taking the electro conductive shaft as the center axis, setting a direction of a spray gun (the center line of jet direction of the mist jetted from the gun) to the center axis of the roller base in 10° through 80°, and moving the spray in the axis direction of the roller base. In the coating of the electro conductive coating material, the uniformed coating can be easily formed on both end phases of the roller by setting to the center axis of the roller in 100° through 170° and using jointly therewith. Therefore, the satisfactory electro conductive coating can be formed more efficiently to the periphery of the shaft roller. Then, the developing roller provided with an electro conductive layer having the thickness of 100 µm was constructed by drying on leaving at a normal temperature or at 50° through 60°C for 5 through 6 minutes. Then, the resistance value was obtained by contacting an electrode having an area of 1 cm² with the periphery of the developing roller and applying a voltage having 10 V to the electrode on connecting the shaft with an amperemeter to measure an electric current, and the obtained values were 10³ through 10⁷ Ω · cm².

(Example 1 of developing agent holder roller)

For forming the developing roller, the similar electro conductive coating material as described above was used except an acrylic urethane type electro conductive coating material was used. And the obtained resistance value were 10⁸ through 10¹⁰ Ω · cm².
The developing roller structured as described above was mounted on the image forming device shown in Fig. 4, and the cleaning characteristics was checked on forming an image. First, the resistance value of the developing roller 4a was experimented on taking notice thereof, and it can be known from the experiment that the satisfactory development cleaning can be obtained when the resistance value is in not more than 1 x 10⁹ Ω · cm², preferably not more than 1 x 10⁷ Ω · cm². Namely, if the resistance value exceeds 1 x 10⁹ Ω · cm² during the development, the fogging will be caused and the image density will be decreased on varying the electric potential on the surface of the developing roller 4a (effective developing bias) by the electric current flowing between the developing roller 4a and the electric power for the development bias 11′. Further, in the range of 1 x 10⁹ Ω · cm² and/or 1 x 10⁷ Ω · cm², the problems on the causing of fogging or the decrease of image density will tend to be slightly caused depending on the parameter of the charged amount of the used toner.
Next, the experiment was carried out by changing the thickness of the surface of the electro conductive layer in the range of 5 through 500 µm for the above mentioned developing agent hold roller (developing roller) 4a, it was confirmed that the thickness is to be set preferably in the range of 20 through 400 µm. Namely, the possibility of damaging the function of the electro conductive layer 20 and of causing the fogging or the density unevenness can be surely reduced.
Further, a sample was formed in view of the smoothness and/or the roughness of the electro conductive layer 20 for the developing agent hold roller (developing roller) 4a, and the characteristics estimation was carried out. As a result, it becomes clear that the ratio not more than 3 µmRz is preferable on the basis of 10 point average roughness defined in the JIS (Japanese Industrial Standard) 0601. Namely, if the ratio exceeds 3 µmRz, the thickness of the adhered toner layer increases, and further the increase of non-charged toner, the fogging and the cleaning failure will be caused as a result. It is desirable that the ratio is not more than 10 µm Rmax when it is defined by the maximum height (Rmax) of the above mentioned JIS-0601.
As the characteristics required for the flexible layer 19 of the developing roller 4a, it can be example such as the hardness, the compression permanent strain, the chemical stability and the adhesion property with the electro conductive layer 20. Namely, the hardness is desired to be soft in view of the object for the working accuracy of the device and the parts and for softening the requirement for the assembling accuracy. It is about 10 through 40 degrees, preferably about 20 through 30 degrees on the basis of the A-type rubber hardness meter in the JIS-6301. Further, the compression permanent strain is desired to be not more than 20%, preferably not more than 10% on the basis of the measuring method in the JIS-6301, namely, the method for measuring the strain amount to define with the percentage (%) on compressing the 25% of thickness of the specimen, leaving for 22 hours at a temperature of 70°C. Here, since the material provided with a flexible layer having thickness of 5 mm arranged on the outer periphery of the shaft having an outer diameter of 8 mm to have the final outer diameter of 18 mm was used as a specimen, the compression of 25% is corresponding to the compression of 5 x 2 x 0.25 = 2.5 mm, and if the compression permanent strain exceeds 20%, the strain will be possibly caused at the compression position of the coating blade to appear as a white line on the image. Further, in view of the chemical stability, this is the most important characteristics for the actual application.
For example, it should be avoided to happen that the dispersedly contained additives are deposited to react with the toner, and the toner is melt to adhere or to react with the sensitive layer to cause the deterioration of the sensitivity. Further, the adhesive property with the surface electro conductive layer is also important. As the flexible material satisfying such points mentioned above, it can be exampled a NBR rubber, a chloroprene rubber, an urethane rubber, a silicone rubber, an ethylene propylene rubber (EPR or EPDM), an urethane type foaming material and a silicone type foaming material, etc. When the silicone rubber is used, a primer treatment is required for obtaining sufficient adhesiveness with the surface electro conductive layer. Further, a plasticity adding agent or a curing agent is not preferably contained when the flexible materials described above are used.
For the toner supply roller 13, an urethane foam having cell constant of 100/25 mm is suited. The material made by mixing above urethane foam with an electro conductive carbon powder to add the electro conductivity acts to loosen a elelctro static cohesion of the toner, therefore, it is suitable for forming the uniformed toner layer. Further, a brush roller or a low hardness rubber roller having the hardness not more than 10 degrees is also applicable. Therefore, the toner supply roller 13 is provided with the contact depth of about 0.1 through 1.0 mm to the developing roller 4a, and is rotated on being set the peripheral speed of about 1/4 through 2 times of the developing roller 4a. Namely, the toner can be supplied even in the case of whole phase black solid development wherein the large amount of the toner is required.
The coating blade 14 controls the amount of the toner adhering to the surface of the developing roller 4a and acts to add the tribo electric charge by means of the frictional electrification, therefore, it is formed of the material which is easily frictional charged. Namely, since toner particle is to be charged in negative in the present invention, it is preferably selected the material positioning at the positive side in the frictional electrification order, for example, a silicone rubber, a polyamide resin, a melamine formalin resin, a polyurethane rubber, a styrene acrylonitrile copolymer, a sheep wool and a quartz, etc. For the actual application, it is required to select the material which can avoid to form the ununiformed toner layer on the surface of the developing roller 4a by adhering the toner 7 to the coating blade 14 even used for a long period of time. As a result from the experiment carried out by the present inventor, when the silicone rubber having a mold release property was used, the adhesion of the toner was not caused after the printing experiment used therein hundred thousand of sheets of A4 size papers, and the toner layer having uniformed thickness was able to be constantly obtained. Further, the toner particles were surely charged in negative, and the adhesion of the toner charged in the reverse polarity was not found in the background part of the obtained image. Furthermore, since the toner layer was thin, the decrease and/or the degradation of the cleaning characteristics was not confirmed.
The formation and the contacting system for the coating layer have some kinds of selection, for example, a process for pressurizing the belly portion of the slab, a process for pressurizing the edge portion of the slab and a process for pressurizing the plane of end portion of the slab. In view of the point in which the uniformed toner layer can be formed constantly by the slight pressure force (control of the adhesion amount), the process for pressurizing the edge portion of the slab is effective. However, if the sharp edge is used as it is, it will be easily caused such disadvantage that the uniformity of the toner is remarkably effected by the quality of the working accuracy of the edge and the mount accuracy of the coating blade 14, and the toner particles passing under the pressure force cannot be sufficiently fictionally charged because of the small contacting area. In view of the point as described above, it is preferable that the edge is worked in circular. Namely, the thin layer can be formed by the light load and the toner can be surely charged by the circular edge. For example, the coating blade 14 made of the silicone rubber having the thickness of 3 mm and the top end portion worked in circular shape to have a diameter of 3 mm was used for the development to control the toner layer held on the surface of the developing roller 4a in the system in which the circular portion is pressed to contact or the belly portion is pressed to contact. The obtained results are shown in the following table.

In the table, the estimation subject A designates an appropriate load (whole load ÷ length of blade), B designates a rotation torque of the developing roller, C designates the amount of toner adhesion (weight of toner adhered to unit area of developing roller surface), D designates the charged amount of the toner, E designates the image density, F designates the fogging (visual estimation for image), and G designates the cleaning characteristics (visual estimation for image).

TABLE

SUBJECT	PRESS CONTACT OF CIRCULAR PORTION	PRESS CONTACT OF BELLY PORTION
A
	10 to 50 g/cm	70 to 150 g/cm
B	800 to 1000 g·cm	1500 to 2500 g·cm
C	0.4 to 0.8 mg/cm²	0.9 to 1.5 mg/cm²
D	-6 to -20 µc/g	-2 to -10 µc/g
E	1.35 to 1.42	1.40 to 1.44
F	O	Δ
G	O	Δ

As can be known from the table mentioned above, in the case in which the control of the toner layer was carried out by pressing to contact the circular portion, the thin toner layer was able to be obtained by the light load. Therefore, the force required for driving the developing roller, namely, the rotation torque is not necessary to be large, thereby, the miniaturization and simplification of the driving system can be achieved. Further, when it was used for a long period of time, the compression permanent strain was not caused, and the white lines in the image was not appeared. Therefore, the circular portion of the coating blade is required to have a radius of about 0.2 through 10 mm, preferably about 0.5 through 5 mm, and a mirable type silicone rubber TSE260 - 7U and TSE270-7U (both are trade name, manufactured by "Toshiba Silicone Co., Ltd.), which is excellent in the abrasion resistance can be exampled.
Next, a concrete example for the image forming method will be described. One component toner containing styrene acrylic resin as a base, and carbon black, a charging control agent and a flow property reforming agent was used for forming an image to the developing device as structured mentioned above. The characteristics inquiry was carried out under the condition as described below. Namely, the toner charged amount was 15 µC/g, the toner adhesion amount on the surface of developing roller 4a was 0.6 mg/cm², an average particle size of the toner was 8 through 9 µm, the particle size dispersion was 1 through 20 µm, the hardness of the developing roller was 30 degrees (JIS A type), the resistance of the developing roller was 1 x 10⁴ Ω · cm², the development nip width was 2.0 mm, the periphery speed of the developing roller was 74 mm/sec (2.0 times of the sensitive body), the protection resistance was 10 MΩ, the development bias voltage was -200 V, the image electric potential of the electro static latent image was -50 V, the non-imaged part electric potential was -500 V, and a corona charge system was used as the transfer device and its transfer efficiency was 60 through 90%.
First, on taking the notice of the image density of the development, when (Vd/Vp)·m was not less than 0.5 mg/cm², the obtained image density was not less than 1.2, and when it was less than 0.5, the obtained image density was lower than 1.2 and was poor image.
In this example, the toner adhesion amount m/(mg/cm²) on the surface of the developing roller 4a and the rotation speed ratio Vd/Vp between the developing roller 4a and the sensitive drum 1 was used as a parameter, then, the image density obtained by developing on varying Vd/Vp within the range of 0.5 through 3.0 in each case wherein the toner adhesion amount m is 0.2 mg/cm², 0.5 mg/cm², 0.8 mg/cm². As a result, it was confirmed that the image density is not determined only by m or (Vd/Vp), it is substantially determined univocal by the product of m and (Vd/Vp). Therefore, for obtaining the satisfactory development, it is required to set (Vd/Vp)·m in not less than 0.5 mg/cm².
On taking the notice of the cleaning characteristics, when the above mentioned (Vd/Vp)·m exceeds 3.0, the cleaning function is deteriorated to appear the ghost.
Namely,

(1) the amount of the toner supplied to the unit area on the surface of the sensitive drum equals (Vd/Vp)·m. Therefore, when the rate of (Vd/Vp)·m is large, the apparent thickness of the toner layer in the developing position becomes larger to fade down the cleaning electric field, thereby, the cleaning characteristics is lowered.
(2) since the amount of the toner supplied to the sensitive drum 1 is large, the surplus development is caused and the toner is adhered to the imaging part in more than necessity amount, and the amount of the remained toner 8′ is inevitably increased. Therefore, a large amount of the remained toner 8′ is to be cleaned up, and the cleaning defection is easily caused.

In view of the truth as mentioned above, in the development cleaning process using one component toner, (Vd/Vp)·m is to be selected in the range of 0.5 mg/cm² through 3.0 mg/cm², preferably in the range of 0.8 mg/cm² through 2.0 mg/cm².
Further, on taking the notice of only the toner adhesion amount m (mg/cm²) on the surface of the developing roller, when it is m<0.2 mg/cm², the cleaning characteristics can be obtained in sufficient, but it is required high rotation speed of the developing roller 4a for obtaining the satisfactory image density, and the abrasion of the developing roller 4a and the tailing of the image are caused. When it is m>1.2 mg/cm², the cleaning characteristics is deteriorated to cause a ghost on the image regardless of the speed of the developing roller.
When the charged amount of the toner is less than 3.0 (µC/g), since the electrostatic suction force (image force) acting between the toner 8 and the surface of the developing roller 4a is faded down, it tends to be caused that the toner particles drops out from the surface of the developing roller 4a and the fogging is caused on the non-imaged part. When it exceeds 30 (µC/g), the above mentioned image force is increased, therefore, the toner amount being transferred to the sensitive drum 1 is decreased to facilitate to cause the decrease of image density. Further, in view of the cleaning, since the resiliency for the remained toner 8′ is increased, the ghost tends to be caused. As a result, the charged amount of the toner is to be preferably set within the range of 3.0 through 30 (µC/g).
On taking the notice of the periphery speed and/or the moving speed of the developing roller 4a and the sensitive drum 1, when the periphery speed of the developing roller 4a is less than 1.5 times that of the sensitive drum 1, the fogging is increased on the background to lower the cleaning characteristics, and the image density becomes easily insufficient. The reason for the increase of such fogging is not clear, but it can be considered as one of the reasons that if the speed difference to the sensitive drum 1 is small, the frictional electrification of the toner particle at the developing position becomes insufficient. When the above mentioned speed rate exceeds 4 times, the toner splash in the circumference of the developing roller 4a increases to cause the possible tailing and fogging is appeared in the image. Therefore, the ratio of the periphery speed and/or the moving speed between the developing roller 4a and the sensitive drum 1 is preferably selected in the range of 1.5 times through 4.0 times.
In the image forming method as described above, the control for the fogging and the cleaning is carried out by the electric field between the non-imaged part in the electro static latent image and the developing roller 4a. Namely, the image having an excellent quality and the sufficient image density without fogging and ghost can be obtained by defining -500 V ≦ Vo-Ve ≦ -100 V, and 50 V ≦ Vq-Ve ≦ 300 V, wherein each value of non-imaged part, imaged part and effective developing bias is defined as Vo, Vq and Ve (each value is negative). Herein, when it is Vo-Ve ≧ -100 V, the cleaning electric field is not sufficient, therefore, the fogging and ghost are easily caused. When it is -500 ≧ Vo-Ve, the cleaning field is too large, therefore, the positive electric charge is injected from the developing roller 4a into the toner particles, and the toner is adhered to the non-imaged part to cause the fogging, and this is resulted remarkably under high humidity atmosphere. In the imaged part, when it is 50 V ≧ Vq-Ve, since the developing electric field is not sufficient, the image density is not sufficient. When it is Vq-Ve ≧ 300 V, the line image becomes thick by the surplus development. Therefore, the relation among each value of non-imaged part, imaged part and effective developing bias, namely, Vo, Vq and Ve (each value is negative) is preferably set as -500 ≦ Vo-Ve ≦ -100 V (preferably -400 V ≦ Vo-Ve ≦ -200 V), and 50 V ≦ Vq-Ve ≦ 300 V (preferably 70 V ≦ Vq-Ve ≦ 200 V).

Example 2

First, the explanation will be regarded to the function of the image forming method by controlling the amount of the remained toner remaining on the latent image phase of the latent image holder (sensitive drum) in the amount of not more than 0.35 mg/cm² after the developed image formed by the same method as in the Example 1 is transferred to the image supporter.
Fig. 6 is a cross-sectional view showing typically the image forming mechanism. The case in which the toner layer 8a made of one component non-magnetic toner is formed on the surface of the developing roller 4a provided with the electro conductive shaft 18, the flexible layer 19 and the electro conductive layer 20, then, it is contacted with the surface of the sensitive drum as the latent image holder 1 to obtain the image by developing and cleaning. As the sensitive drum 1, it can be used the positive electrification type such as the selenium type and the negative electrification type formed of zinc oxide or organic photoconductive material. Here, it will be explained the case in which the latent image is formed by image exposing on the organic sensitizing body of the negative electrification type, and the reverse development is carried out to the obtained latent image by the negative electrification toner 8 and the remained toner 8′ on the sensitive drum 1 is cleaned up simultaneously. The electro conductive surface layer 20 of the developing roller 4a is connected to the developing bias electric power 11′ through the protection resistance 11a′ and the applied with the developing bias of the voltage V_b.
The territory analysis will be carried out by modeling the development area in Fig. 6 as shown in Fig. 7. The Gauss'law is applied to each layer of Fig. 7.
d ivD_p = 0
d ivD_r = ρ_r
d ivD_t = ρ_t
The boundary condition are as follows on defining the unit normal line vector in x axis as n:
D_p · n = σ_b
(D_r - D_p)·n = σ_p
(D_t - D_r)·n = O
-D_t·n = σ_t
ø_p(0) = 0
ø_p(d_p) = ø_r(d_p)
ø_r(d_p + d_r) = ø_t(d_p + d_r)
ø_t(d_p + d_r + d_t) = V_b
When the surface electric potential of the sensitive layer before reaching to the development area is defined as V₀;
σ_p = ε_p V₀/d_p
And when the toner electric charge is converted from the volume electric charge densities ρ_r and ρ_t into the weight electric charge densities q_r and q_t;
ρ_r = q_r/d_rm_r,
ρ_t = q_t/d_tkm₀
Herein, the symbols in Fig. 7 are used. The symbol k designates the speed ratio given from k = V_r/V_p, when each periphery speed of the developing roller 4a and the sensitive drum 1 is defined as V_r and V_p. The symbol m₀ designates the toner adhesion amount on the surface of the developing roller 4a, and its unit is kg/m².
On achieving the above mentioned problem on the boundary value, and obtaining the electric field dø_r/dx and defining the value of x when it is dø_r/dx = 0 as x₀, the remained toner layer is separated at the position of x = x₀ in the last step of development, thereby, it is separated into the sensitive drum 1 side and the developing roller 4a side. The amount of the remained toner m after cleaning can be expressed by the formula as mentioned below on using the result from the above mentioned boundary value problem.
m = m_r (x₀ - d_p) / d_r
= 1/A {(d_r/2ε_r + d_t/ε_t) m_r
- (V₀ - V_b) / q_r
+ 1/2 q_t / q_r·d_t /ε_t k m₀}
wherein, A = d_p/ε_p + d_r/ε_r + d_t/ε_t. The result obtained by calculating the cleaning characteristics on inserting the experimental value into the above mentioned formula is shown by the dashed lines in Fig. 8. Fig. 8 shows the variation of the toner amount m on the sensitive drum 1 after developing and simultaneous cleaning by the amount m_r of the remained toner 8′ after transfer.
V₀ - V_b = -200 V
d_p = 20 µm, d_r = 11 µm, d_t = 11 µm
ε_p = 3.4 ε₀, ε_r = 1.0 ε₀
ε_t = 1.0 ε₀ ( ₀: dielectric constant in vacuum)
q_r = -3.1 x 10⁻² C/kg
q_t = -1.26 x 10⁻² C/kg
m₀ = 4.3 x 10⁻³ kg/m²
k = 2.0
wherein, the toner amounts m and m_r on the sensitive drum 1 were obtained by measuring the weight of the sensitive drum after the adhesion of the toner. The toner electrification amount q_r and q_t were calculated by measuring the amount of the electric charge flowing into the Coulomb's meter connected to the electro conductive base of the sensitive drum when the toner on the sensitive drum was blown by the air.
The physical meaning shown by the dashed lines is as described below. Since the electric potential condition V₀ - v_b = -200 V is corresponded to the non-imaged part, the remained toner 8′ is to be completely cleaned up under the condition mentioned above. Namely, the region m = 0 becomes the proper region in which the memory is not generated. The result from the above theory analysis indicates that the cleaning can be completely carried out if the amount m_r of the remained toner 8′ after the transfer is not more than 0.23 x 10⁻² kg/m². Further, in view of the harmonization with the result from the experiment described in the figure, it is remarkably satisfactory, therefore, it can be considered that the theory analysis as described above is appropriate.
The symbols q_t, m₀ and k among the above mentioned experimental values, are the parameters which are relatively easily varied by the material of the toner and the setting condition of the image forming device. On varying these parameters within the practically variable range (q_t = -0.2 x 10⁻² through -2.5 C/kg, m₀ = 2.0 x 10⁻³/8.0 x 10⁻³ kg/m², k = 1.2 through 3.5), and the theory curve was calculated. As a result, the cleaning can be carried out until 0.35 x 10⁻² kg/m² (= 0.35 mg/cm²) at maximum in accordance with the condition. Therefore, the sufficient image without remaining the cleaning to the non-imaged part (namely, without positive memory) can be obtained On setting the remained toner 8′ in not more than 0.35 mg/cm², preferably not more than 0.23 mg/cm².
In Fig. 8, the results from the experiment regarding a half tone and a solid image are indicated as well as the characteristics in the above mentioned non-imaged part. The solid image corresponds to the part in which the electric potential of the sensitive drum 1 is sufficiently damped by the image exposure as the latent image. Therefore, if the amount of the remained toner 8′ is excessively large, the damping of the electric potential is tented to be inhibited by the light cutoff action to decrease the developing toner amount (namely, negative memory is generated). It can be known from Fig. 8, the amount of the remained toner 8′ is preferably set in not more than 0.5 x 10⁻² kg/m² for keeping the amount of the developing toner in not less than 0.8 x 10⁻² kg/m².
The half tone image corresponds to the intermediate electric potential condition between the imaged part electric potential and the non-imaged part electric potential, therefore, it has low development electric field or cleaning electric field and the memory is hardly generated. However, the latent image formed of the aggregation of the mesh point image and the fine line is also regarded as the half tone image when it is the intermediate electric potential to the macro. In concrete, the half tone image region is defined as the region having an average value between the images in not more than 0.5 mm. In Fig. 8, among the various half tone images, it is selected the half tone image in which the memory is notably appearing, and the characteristics thereof is shown. It can be known from Fig. 8, that the negative memory or the positive memory is appeared when the remained toner 8′ exceeds 0.1 x 10⁻² kg/m². Therefore, when the half tone image is included, the generation of the memory can be controlled by setting the amount of the toner 8′ in not more than 0.1 mg/cm², preferably not more than 0.04 mg/cm².
Fig. 9 is a cross-sectional view showing the main structure of the image forming device utilized in the present example. Numeral 1 designates the sensitive drum corresponding to the latent image holder, the organic sensitive body of the negative electric charge is used in this example, ahd this sensitive drum 1 is charged in the corona electric charge by the electrification device 2. The latent image is formed by the exposure of the light beam, such as the laser beam, from the image modulated exposure device 3. As the developing device 4, it is used a system for forming the thin layer of non magnetic toner on the developing roller 4a′ by pressuring the coating blade 14 on the surface of the developing roller 4a′ provided with the electro conductivity and the flexibility. Therefore, the developing roller 4a′ is pressurized to the sensitive drum 1 on keeping the nip width of 2 through 3 mm, and rotates at the surface speed in the range of 1.2 through 4.0 times of the sensitive drum 1. As the developing roller 4a′, it is utilized in which the flexible layer 19 having the rubber hardness of 15 through 40 degrees and the electro conductive layer 20 having the resistance of not more than 10⁷ Ω · cm are provided in order on the periphery of the metal shaft 18, or the derivative layer having the thickness in the range of 20 through 100 µm provided on the surface of the flexible layer having the electro conductivity (not less than 10¹¹ Ω · cm). At the contacting position between the developing roller 4a′ and the sensitive drum 1, the development is carried out simultaneously with the cleaning as described above. The electro potential on the developing roller 4a′ is preferably set in the range of -150 through -400 V, the electro potential on the non-imaged part of the sensitive drum 1 is preferably set in the range of -300 through -600 V, and the electro potential of the imaged part is preferably set in the range of 0 through -150 V.
The present invention is not limited only to the examples as described above, for example, the image forming method utilizing the jumping method disclosed in Japanese Patent Publication No. 32375, 1983 and U.S.P. No. 4,342,822 etc., and the FEED developing method disclosed in Japanese Patent Publication No. 35984, 1988 and Japanese Patent Application Laid Open No. 176961, 1986 are able to be included in the present invention. Further, the present invention is applicable to all the methods for forming the image by contacting the thin toner layer composed of non magnetic or magnetic toner in general.

Claims

1. An image forming method, comprising the steps of:
developing a latent image (imaged part) formed to be held on a latent image holder by contacting a developing agent holder holding a layer formed of one component developing agent with said latent image holder;
transferring said developed image on an image supporter; and
cleaning a developing agent adhered to a non-imaged region of said latent image holder after said step of transferring simultaneously with said step of developing,
wherein said steps of developing and cleaning are carried out under the condition expressed by the following formula:
0.5 ≦ (Vd/Vp)·m ≦ 3.0
wherein a moving speed of the surface of said developing agent holder is defined as Vd, a moving speed of said latent image holder is defined as Vp, and a developing agent adhering density on the surface of said developing agent holder is defined as m (mg/cm²).

2. The image forming method according to claim 1, wherein said developing agent adhering density m on the surface of said developing agent holder is set in the range of 0.2 through 1.2 mg/cm².

3. The image forming method according to claim 1, wherein said developing agent adhering density m (mg/cm²) on the surface of said developing agent holder is set in the range expressed by the following formula:
0.8 mg/cm² ≦ (Vd/Vp)·m ≦ 2.0 mg/cm².

4. The image forming method according to claim 1, wherein said developing agent holder is an elastic roller, and the absolute value of electric charge amount of said developing agent layer formed on the surface of said developing agent holder is set in the range of 3 through 30 µC/g, the moving speed Vd of the surface of said developing agent holder is set in the range of 1.5 through 4.0 times of the moving speed Vp of the surface of said latent image holder, the absolute value of electric potential difference between the surface of said developing agent holder and said non latent imaged part (non-imaged part) is set in the range of 100 through 500 V, and the absolute value of electric potential difference between the surface of said developing agent holder and said latent image holder (imaged part) is set in the range of 50 through 300 V.

5. The image forming method according to claim 4, wherein said elastic roller is formed with an elastic layer coaxially provided on the periphery of metal shaft, and an electric resistance value between the surface of said elastic layer and said metal shaft is not more than 1 x 10⁷ Ω · cm².

6. An image forming method, comprising the steps of:
developing a latent image (imaged part) formed to be held on a latent image holder by contacting a developing agent holder holding a layer formed of one component developing agent with said latent image holder;
transferring said developed image on an image supporter; and
cleaning a remained developing agent after transfer adhered to a latent image phase of said latent image holder after said step of transferring simultaneously with said step of developing,
wherein the amount of said remained toner is set in not more than 0.35 mg/cm².

7. The image forming method according to claim 6, wherein the amount of said remained toner is set in not more than 0.23 mg/cm².

8. The image forming method according to claim 7, wherein the amount of said remained toner is set in not more than 0.1 mg/cm².

9. The image forming method according to claim 6, wherein the surface electric potential of said developing agent holder is set in the range of -150 through -400 V, the electric potential of said non latent imaged part of said latent image holder is set in the range of -300 through -600 V, and the electric potential of said latent image part of said latent image holder is set in the range of 0 through -150 V.

10. The image forming method according to claim 6, wherein a uniforming brush is contacted with said latent image holder after the step of transferring to uniform and dissipate said remained toner adhered to said latent image holder.